The present invention relates to a magnetic disk apparatus which positions its magnetic head at a target track by using servo information recorded on a magnetic disk.
A magnetic disk apparatus of this kind is described in U.S. Pat. Nos. 5,053,899 and 5,023,732, assigned to the same assignee as the present invention. A magnetic disk, which is a recording medium of the magnetic disk apparatus, has data areas and servo areas alternately arranged in a circumferential direction along concentric circular tracks. In each of the servo areas, a servo identification (ID) section, in which a predetermined pattern signal is recorded at a different recording frequency from that of the data areas, and a servo information section are arranged in that order.
After the magnetic disk apparatus positions its magnetic head roughly at a target track in a track seek mode, the apparatus shifts to a track following mode, in which the apparatus reads with its magnetic head two adjoining servo information signals from the servo information section in the servo area so as to generate a positional error signal by determining the difference between the two servo information signals. This positional error signal is used for positioning the magnetic head finely on the center line of the target track.
In order to detect the position of the servo information section on the target track, the aforementioned servo ID section is provided. The magnetic disk apparatus compares a voltage of a reproduced pattern signal from the servo ID section with a predetermined threshold, and generates timing pulses triggered by a time point at which the voltage of the reproduced pattern signal surpasses the threshold. The apparatus samples the reproduced servo information signals read out by the magnetic head in synchronism with the timing pulses, and thereby detects the value of the servo information signals.
In the conventional magnetic disk apparatus, when the voltage of the reproduced pattern signal from the servo ID section varies, the timing at which this voltage surpasses the threshold, i.e. the starting time point for generating the timing pulses, also varies, causing a variation in the output timing of the timing pulses. Accordingly, the servo information should be recorded in a sufficiently large area to allow for the range of this timing variation. This means that the servo information section is sufficiently large and, consequently, requiring long servo areas in the tracking direction. As a result, the data areas are correspondingly reduced, so as to reduce the data recording capacity.